1. Field of the Invention
The present invention relates to vacuum blankets and more particularly, to vacuum blankets of the type used for holding a pre-sensitized sheet of film material in precise registration with a film master against a flat, transparent, rigid plate for controlled light exposure to form a pattern or image on the pre-sensitized sheet of film.
2. Description of the Prior Art
Various types of vacuum contact systems and blankets have been developed for use in the graphic arts and one such vacuum contact system is shown in the Fosh Patent No. 4,669,870. German Patent No. 29 28 320 C2 discloses a vacuum blanket having crisscrossed grooves for facilitating air removal from the central portion of the blanket when a vacuum is drawn. German Patent No. 31 22 707 Al discloses a blanket backed up by a mechanical roller system for eliminating entrapped air. Maher et al U.S. Pat. No. 4,551,016 discloses a vacuum printer wherein a spring biased floating plate is used to back up a flexible blanket and Maher et al Patent No. 4,484,813 discloses a system wherein a perforated rigid plate backs up a flexible blanket.
Problems have been present with many types of vacuum blankets because of air pockets remaining between film sheets when a vacuum is drawn and often-times a vacuum blanket temporarily seals around the periphery first, making it difficult to remove interior air pockets even though a high vacuum is drawn on the space between a rigid plate of glass or transparent sheet material and the pre-sensitized and master film sheets.
Many vacuum blankets of the prior art require an exceptionally long interval of time to insure the complete removal of such air pockets and this results in a much slower operating speed with attendant higher costs for the production of graphic arts materials.
In many prior art systems the registration between a film master and a pre-sensitized sheet has been less than precision due to the presence of small air pockets and/or ridges commonly formed while evacuating the air existing between the film media sheets. As shown in the aforementioned patents, some units have used mechanical means such as springs, rollers, etc., to press the media sheets together into registration against a transparent rigid surface. Some systems require an elaborate and expensive control system for providing variable levels of vacuum in different zones over the face of the film sheets. Even such complex systems have still been subject to the aforementioned difficulties and have been costly to produce and relatively slow in operation with attendently a lower grade or quality of reproductions.